Theresa May will slap down Britain's most senior police officer in a row over the use of stop and search powers.
Claims that knife crime is rising as a result of curbs on the policing method are "simply not true", the Home Secretary will say.
The comments put her publicly at loggerheads with Metropolitan Police commissioner Sir Bernard Hogan-Howe, who earlier this year suggested the reforms were behind a dramatic increase in knife crime in London.
Mrs May will also tell the National Black Police Association conference that more must be done to boost the number of women and ethnic minority officers as she rounds on four forces for having no black officers on the payroll.
Officers must resist making a "knee-jerk reaction on the back of a false link" over stop and search powers and the impact they are having on knife crime, she will say.
It is "simply not true that knife crime is rising because the police are no longer stopping and searching those carrying knives," the conference will be told.
Most of the reduction in Met Police stop and searches relates to suspicions over drug or property crimes, with "blade" stops accounting for less than 1%.
When reforms of the system were announced in 2014 just one in 10 stop and searches led to an arrest and black people were seven times more likely to be stopped than whites.
Mrs May will say: "Stop and search is an important police power – and I will always back police officers who use their powers legitimately and accountably. But when stop and search is misapplied, and when people are stopped and searched for no good reason, it is unfair, it wastes valuable police time, and it damages the relationship between communities and the police.
"I know there are those who say that our reforms have gone too far, that the pendulum has swung too much the other way, and that reforms to stop and search are linked to knife crime in our capital and elsewhere.
"But to them I say this: stop and search reform has worked, it must continue, and – if you look at the evidence – it shows no link whatsoever with violent crime."
Latest figures show no force has a black and minority ethnic representation in line with the local population and four forces have no officers. Women make up just 28.2% of all police officers.
Mrs May will say: "Incredibly, four forces do not employ any black or black British police officers at all, and female officers make up 28% of all police officers but 51% of the total population.
"This comes on top of existing statistics showing that there are only two BME chief officers in England and Wales, and 11 forces have no BME officers above chief inspector rank.
"This is simply not good enough. I hope these figures will provide chief constables with the information they need to identify areas for improvement and for the public and PCCs to hold them to account."